1. Field of the Invention
The invention relates to sodium hyaluronate and more particularly to a composition of sodium hyaluronate which is useful as an aid in ophthalmic surgery.
2. Brief Description of the Prior Art
Sodium hyaluronate (NaHA) viscoelastic compositions have been employed extensively in certain ophthalmological procedures such as cataract surgery. In these procedures, the composition may be placed in the anterior chamber of the eye to facilitate surgical manipulations. Generally, the composition is partially or substantially completely removed at the conclusion of surgery; see for example U.S. Pat. No. 4,328,803 where a partial removal by dilution in-situ is suggested.
With some frequency, surgeons have observed an increased intra-ocular pressure (IOP) following surgery associated with the use of viscoelastic compositions as a surgical aid. The reason for the increase is not fully understood since transient increases occur following even routine surgical procedures where viscoelastic aids are not employed; see for example Ruiz, R. S. et al., Amer. J. of Ophthal., 103: 487-491, (1987). However, it has been suggested that IOP elevations associated with the use of a viscoelastic surgical aid composition occur when the outflow of aqueous secretions through the trabecular network is reduced. It has also been suggested that this obstruction in outflow is effected by the presence of residual molecules of the viscoelastic, exemplified by sodium hyaluronate. The relatively large sodium hyaluronate molecule may reduce flow of the aqueous secretions, when they enter the trabecular network.
Obstruction of the trabecular network may also be related to the viscosity of the viscoelastic composition, when it enters the trabecular network; see Benedetto, Ophthal. Times, April 15, 1987, p. 58. Schubert, et al., (Exp. Eye Res., 39: 137-182, 1984) reported that lower viscosity NaHA solutions (10,000 centistokes) resulted in consistently higher IOP elevation than higher viscosity solutions (40,000 cSt). They reasoned that this occurred due to the rapid dissolution of this NaHA solution causing a "traffic jam" in the trabecular network.
The importance of both viscosity and molecular weight in NaHA compositions used as aids in ophthalmological surgery, as a possible factor in post-operative IOP increases is suggested by Schubert et al., Exp. Eye Res. 39: 137-152 (1984).
Regardless of the possible mechanisms of post-operative IOP increases, the phenomena is undesired and much attention has been given to a solution to this problem, particularly as it may relate to the use of sodium hyaluronate compositions; see for example the solution posed by Pape in U.S. Pat. No. 4,328,803, who suggests an in-situ dilution of anterior chamber emplaced sodium hyaluronate.
We have discovered that a particular sodium hyaluronate composition of a particular molecular weight and viscosity may be used in ophthamological surgical procedures, and its use is associated with significantly lower increases in post-operative intra-ocular pressures (compared to use of heretofore commercially available sodium hyaluronate compositions).